As is well-known, the clamp-on Doppler ultrasonic flow velocity profile meter measures a flow velocity profile or a flow rate of fluid by measuring the velocities of suspended particles or bubbles contained in the fluid, on the assumption that the suspended particles or bubbles move at the same velocity as that of the fluid. Referring to FIG. 14, which illustrates the operation principle of a Doppler ultrasonic flow velocity profile meter, an ultrasonic wave transducer 11 is secured and positioned to an outer surface of a pipe 21 at an angle relative to the pipe 21 with a sound wave propagative wedge 31. From the ultrasonic wave transducer 11, an ultrasonic wave pulse with a fundamental frequency of fO is transmitted to the pipe 21 at an angle of incidence θw. The incident ultrasonic wave pulse is reflected by reflectors 23, such as suspended particles, in a fluid 22 with a frequency of an echo shifted from the fundamental frequency, depending on the moving velocity of the reflectors 23 (flow velocity of the fluid), namely based on the Doppler effect. A Doppler shift frequency fd of the echo in this case is expressed by the following expression (1):fd=(2·Vf·sin θf·fO)/Cf  (1),where Vf is the flow velocity of the fluid 22, θf is an angle of refraction of the ultrasonic wave at the boundary plane between the pipe 21 and the fluid 22, and Cf is the sound velocity in the fluid 22.
Therefore, the flow velocity V of the fluid 22 can be obtained by the following expression (2). The flow velocity Vf and the Doppler shift frequency fd, each being a function of a position x along the radial direction, are expressed as Vf(x) and fd(x), respectively:Vf(x)=(Cf·fd(x))/(2·sin θf·fO).  (2).
Referring to FIG. 15, which illustrates the principal part of the flow velocity profile meter shown in FIG. 14 and a flow velocity profile at position x in the pipe 21. From the expression (2), current velocities Vf on a measuring line ML of the ultrasonic wave pulse are measured at specified intervals to obtain a flow velocity profile. The obtained profile is integrated about the cross sectional area A of the pipe 21 and is expressed in the following expression (3) to obtain the flow rate of the fluid 22:Q=∫Vf·dA  (3).
FIG. 16 illustrates the entire arrangement of the clamp-on Doppler ultrasonic flow velocity profile meter (a block diagram showing the ultrasonic wave transducer 11 and an inner arrangement of a converter 18 connected to the transducer 11). The arrangement is substantially the same as that of, for example, the Doppler ultrasonic flow meter shown in FIG. 1 of JP-A-2000-97742. Referring to FIG. 16, a transmission and reception timing control unit 12 controls the transmission timing of an ultrasonic wave pulse and reception of the echo. The transmission and reception timing control unit 12 controls a transmitted pulse generating unit 13 to produce a pulse signal for generating an ultrasonic wave pulse transmitted from the ultrasonic wave transducer 11. The ultrasonic wave transducer 11 also receives an echo. A signal due to the received echo is amplified by a received signal amplifying and controlling unit 14. The amplified received signal is subjected to analog to digital conversion at an A/D converting unit 15 according to an A/D sampling clock from the transmission and reception timing control unit 12. The digitized signal is subjected to an operation according to the above expression (2) at a flow velocity profile operation unit 16, to obtain the flow velocity profile. The obtained flow velocity profile is further subjected to the operation according to the above expression (3) at a flow rate operation unit 17, to obtain the flow rate.
According to the above-explained principle, it is possible for the flow velocity Vf and the flow rate Q of the fluid 22 to be actually calculated with the expressions (2) and (3) without depending on the transmission frequency fO of the ultrasonic wave pulse. The present inventors, however, found that the difference in transmission frequency fO of an ultrasonic wave changes the obtained flow velocity Vf and the flow rate Q. In particular, such frequency dependence becomes remarkable when the pipe 21 is made of thin metallic material, while the frequency dependence becomes small when the pipe 21 is made of plastic.
Moreover, in an ultrasonic flow velocity profile meter disclosed in Japanese Patent Application No. 2003-396755, an ultrasonic wave transducer is secured to a wedge to position it at an angle relative to the pipe by taking an angle of incidence of an ultrasonic wave incident on the pipe from the wedge as being no less than the critical angle for a longitudinal wave in the pipe and no more than the critical angle for a shear wave in the pipe. This is provided so that only a shear wave is propagated in the pipe when the sound velocity of the shear wave of an ultrasonic wave propagating in the pipe is equal to or greater than the sound velocity of the longitudinal wave in the wedge (when a metallic pipe is used). According to the flow velocity profile meter, the echo from the reflectors in the fluid to be measured becomes a wave due to only the shear wave propagating in the pipe before being incident on the fluid. Thus, the echo due to the longitudinal wave is not received by the transducer to reduce acoustic noises. However, the problem of frequency dependence of the above-described flow velocity Vf and the flow rate Q is left unsolved.
Accordingly, there still remains a need to solve the above problem and provide an apparatus and method for measuring a flow velocity profile that has a small frequency dependence, while capable of measuring the flow velocity and the flow rate with a higher accuracy. The present invention addresses this need.